The ultimate goal of the research proposed here is to define the molecular mechanism underlying thi control of telomere length in human cells. Telomere length control is of particular interest within the context of human cancer since the activation of a telomere maintenance system is a key step in tumorigenesis. In the preceding funding period we established TRF1 as a key regulator of telomere length in humai cells. We also identified the TRF1 -interacting factor tankyrase and showed that this telomeric PARP function as an upstream negative regulator of TRF1. Our preliminary evidence suggests a role for the third componer of the TAFt complex, TIN2, in regulating the TAF1-tankyrase interplay. Here we propose three sets c experiments (AlMs 1-3) to further dissect the way the TRF1/tankyrase/TIN2 network modulates telomer maintenance by telomerase. Additional components of the TRF1 complex will be sought in AIM 4 and their rol in telomere length homeostasis will be addressed using the tools we have developed in the preceding fundin! period. We also showed that the TRF1 paralog, TRF2, contributes to telomere length control and identified TRF2-interacting protein that is the ortholog of the main telomere length regulator in budding yeast. Th discovery of this protein, hRapl, and its remarkable divergence from budding yeast Rapi p raises the question of the role of both TRF2 and hRap 1 in telomere length control, an issue that will be addressed in AIM 5. The experiments proposed in this renewal application are based on the assumption that the cis-acting mechanism of telomere length control in human cells will be primarily mediated by the two main duple telomeric DNA binding proteins, TRF1 and TRF2. Our rationale is that by focusing on the complexes formed b TRF1 and TRF2, we should be able to gain insight into the key features of the telomere length control circuil Given the role of telomere dynamics in human cancer, understanding the regulation of human telomer maintenance is of central importance.